1. Technical Field
The present disclosure relates to a micro-electro-mechanical gyroscope with open-loop reading device and a corresponding method for controlling a micro-electro-mechanical gyroscope.
2. Description of the Related Art
As is known, the use of micro-electro-mechanical systems (MEMS) has encountered an increasing development in several sectors of technology and has yielded encouraging results, especially in the production of inertial sensors, micro-integrated gyroscopes, and electro-mechanical oscillators for a wide range of applications.
MEMS systems of this type are usually based upon micro-electro-mechanical structures having at least one mass, which is connected to a fixed body (stator) through springs and is movable with respect to the stator according to pre-determined degrees of freedom. The movable mass and the stator are capacitively coupled by a plurality of respective comb-fingered and mutually facing electrodes so as to form capacitors. The movement of the movable mass with respect to the stator, for example on account of an external stress, modifies the capacitance of the capacitors. From here it is possible to trace back to the relative displacement of the movable mass with respect to the fixed body and hence to the force applied. Instead, by supplying appropriate biasing voltages, it is possible to apply an electrostatic force to the movable mass in order to set it in motion. In addition, in order to provide electro-mechanical oscillators, the frequency response of inertial MEMS structures is exploited, which is typically of a second-order low-pass type, with a resonance frequency. By way of example, FIGS. 1 and 2 show the evolution of the magnitude and phase of the transfer function between the force applied to the movable mass and displacement thereof with respect to the stator in an inertial MEMS structure.
MEMS gyroscopes, in particular, have a more complex electro-mechanical structure, which includes two masses, movable with respect to the stator and coupled to one another so as to have one relative degree of freedom. The two movable masses are both capacitively coupled to the stator. One of the masses is dedicated to driving and is maintained in oscillation at the resonance frequency. The other mass is driven in the oscillatory motion and, in the case of rotation of the microstructure with respect to a pre-determined gyroscopic axis with an angular velocity, is subject to a Coriolis' force proportional to the angular velocity itself. In practice, the driven mass operates as an accelerometer, which enables detection of the Coriolis' force and acceleration and hence tracing-back to the angular velocity.
For proper operation, a MEMS gyroscope requires, in addition to the microstructure, a driving device, which has the task of maintaining the movable mass in oscillation at the resonance frequency, and a device for reading the displacements of the driven mass, according to the degree of freedom of the driving mass. Such displacements, in fact, indicate the Coriolis' force and, consequently, the angular velocity and can be detected through electric reading signals correlated to the variations of the capacitive coupling between the driven mass and the stator. As a result of driving at the resonance frequency, the reading signals, caused by the rotation of the gyroscope and correlated to the angular velocity, are in the form of dual-side-band, suppressed-carrier signals (DSB-SC; the carrier is in this case the velocity of oscillation of the driving mass and has a frequency equal to the mechanical resonance frequency).
Since, however, the MEMS gyroscope has a complex structure and the electro-mechanical interactions between the movable masses and stator are frequently non-linear, the useful signal components are frequently superimposed on spurious components, which are not significant for measuring the angular velocity. The spurious components can be due to several causes. For example, reading the capacitance between the movable masses and the stator inherently perturbs the forces caused by driving and by rotation of the microstructure (producing the so-called phenomenon of “electrostatic softening”, which in practice modifies the resonance frequency of the micro-electro-mechanical structure). The charge displacements induced and detected, in fact, modify the electrostatic forces between the capacitively coupled elements and affect the dynamics of the system. Other causes of disturbance, which are practically impossible to eliminate, are the imperfections of production and the process dispersions, so that the behavior of real devices differs in a way that is only statistically predictable from the design. A very common defect, for example, depends on the fact that the mass used for driving oscillates in a direction not perfectly coinciding with the degree of freedom envisaged in the design stage. In this case, the defect of driving affects the useful signal, introducing a component of unknown amplitude at the same frequency as that of the carrier and 90° out of phase.
On the other hand, the amplitude of the disturbance components is in many cases significant and cannot be simply neglected without introducing unacceptable distortions.